Monday, March 05, 2018

bioRxIv: Humidity As A Non-pharmaceutical Intervention For Influenza A


For centuries the Chinese have used vinegar as a disinfectant, and sometimes boil it in their homes to ward off respiratory ailments, such as influenza. The noxious odor was believed to `purify'  the air inside the home, although sometimes with tragic results (see SCMP Two dead after boiling white vinegar).
A spike in the sale of vinegar in China in 2002-2003 was credited as an early sign of their growing (SARS) epidemic (cite).
While the use of vinegar was unlikely to add any beneficial effect, vinegar is 95% water, and boiling it undoubtedly raised the humidity inside their homes.
And that, as we've discussed often in the past, could impact the survivability  and transmissibility of the virus.
In 2008 researchers Jeffrey Shaman and Melvin Kohn established a correlation between the AH (Absolute Humidity) and the survival, and transmission of the influenza virus (see It's Not So Much The Heat, It's The Humidity).

A 2012 study (see Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum) found both extremely low and extremely high humidity were conducive to flu transmission – at least when it resides in mucus and respiratory fluids like those found in your nose, throat, or lungs.

Essentially, these researchers inoculated droplets of simulated respiratory fluids (containing salts & proteins) with influenza viruses, and tested their survivability at different humidity levels.
  • At low humidity (< 50%) the droplets evaporated quickly, and the virus survived well in a dry environment.
  • At high humidity (near 100%), the droplets were stable, and the virus survived as well.
But at humidity levels in-between, the droplets slowly dried out, increasing the concentration of salts and proteins to which the viruses were exposed, decreasing their survival rate.

In 2013, in NIH Study: Climate & Influenza Transmission, we looked at a PLoS One study called High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs that concluded that `. . . maintaining indoor relative humidity at levels greater than 40% can significantly reduce the infectious capacity of aerosolized flu virus.'
The irony here is that hospitals are normally kept cool and dry in order to curb the growth of bacteria, but may be unintentionally providing an environment conducive to to the spread of respiratory viruses like influenza, SARS & MERS.
Also in 2013, a study called Stability of Middle East respiratory syndrome coronavirus (MERS-CoV) under different environmental conditions by N van Doremalen, T Bushmaker, V J Munster found, under favorable temperature and humidity conditions (such as you might find in an air conditioned hospital), the MERS virus survives quite well on surfaces, and in the air.
Which could go a long way towards explaining why both MERS and SARS are notorious for burning through hospital wards (see The Lancet: Mapping The Korean MERS-CoV Superspreading Event).
While largely anecdotal, the evidence suggests that raising the humidity inside homes, offices, and health care facilities might be a reasonably effective NPI (Non-pharmaceutical Intervention) during times of severe influenza activity, such as during a pandemic.
Although great in theory, real world testing of influenza virus survival and transmissibility under varying relative humidity levels has been limited.  
Over the weekend, however, a study - that while not yet published in a peer-reviewed journal - has been offered as a preview on the BioRxiv (Bio Archive) site. A h/t to @greg_folkers who tweeted the link.

First a link to the full study, and an excerpt from the abstract, then I'll return.
Humidity as a non-pharmaceutical intervention for influenza A

Jennifer M Reiman, Biswadeep Das, Greg Sindberg, Mark D Urban, Madeleine Hammerlund, Han Lee, Kate Spring, Jamie Lyman-Gingerich, Alex Generous, Tyler Koep, Kevin Ewing, Phil Lilja, Felicity Enders, Stephen Ekker, W. Charles Huskins, Hind Fadel, View Christopher Pierret

This article is a preprint and has not been peer-reviewed


Influenza is a global problem infecting 5-10 % of adults and 20-30 % of children annually. Non-pharmaceutical interventions (NPIs) are attractive approaches to complement vaccination in the prevention and reduction of influenza. Strong cyclical reduction of absolute humidity has been associated with influenza outbreaks in temperate climates.
This study tested the hypothesis that raising absolute humidity above seasonal lows would impact influenza virus survival and transmission in a key source of influenza distribution, a community school. Air samples and objects handled by students (e.g. blocks and markers) were collected from preschool classrooms. All samples were processed and PCR used to determine the presence of influenza and its amount. Additionally samples were tested for their ability to infect cells in cultures.
Deliberate classroom humidification (with commercial steam humidifiers) resulted in a significant reduction of the total number of influenza positive samples (air and fomite), viral copy number, and efficiency of viral infectivity. This is the first prospective study suggesting that exogenous humidification could serve as a scalable NPI for influenza or other viral outbreaks.
        (Continue . . . .)

Although you'll want to read the full study in order to understand the methods and materials used, the gist of their findings can be summed up by the following passage from their discussion section:
An increase in average AH from 6.33 mb in control rooms to 9.89 mb in humidified rooms (RH ~42-45 %) was associated with a significant decrease in influenza A virus presence in fomite and air samples in humidified rooms compared to control rooms. Additionally, PCR-positive samples from humidified rooms exhibited lower infectivity than samples from control rooms.
During the opening months of the next pandemic pharmaceutical interventions (vaccines & antivirals) may be unavailable, ineffective, or plagued by manufacturing delays, and so the  immediate goal will be to slow the spread of the virus and to limit its impact through the use of NPIs (see Community Pandemic Mitigation's Primary Goal : Flattening The Curve).

The CDC’s Nonpharmaceutical Interventions (NPIs) web page defines NPIs as:

Nonpharmaceutical interventions (NPIs) are actions, apart from getting vaccinated and taking medicine, that people and communities can take to help slow the spread of illnesses like influenza (flu). NPIs are also known as community mitigation strategies.
To be the most effective they can be, we need to have a keen understanding of the both mechanics of viral transmission, and the effectiveness of specific countermeasures.
While there is much to be learned, the evidence over the past decade suggests that the traditional Chinese practice of raising the humidity inside the home might actually help reduce the spread of respiratory infections among household members. 
Something which may be worth considering during the next flu season or pandemic.  For more on the environmental  factors that may affect virus viability, you may wish to revisit:
Sci. Ttl. Enviro.: Cold-Dry Days Favor H7N9 Transmission

EID Journal: Evidence-Based Options for Controlling Respiratory Virus Transmission
Formidable Flu Fomites

IDWeek: Persistence Of MERS-CoV On Hospital Environmental Surfaces
Study: Survival Of Aerosolized Coronavirus In The Ambient Air 

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